1. Field of the Invention
The present invention relates to fiber optic cables.
2. Description of Related Art
In the field of fiber optic sensors, there are a number of techniques for measuring temperature and strain. Two of the most well known techniques employ fiber Bragg gratings and Brillouin backscatter. Fiber Bragg gratings are devices, inscribed into an optical fiber, which have spectral characteristics (e.g., transmission and reflection) that are dependent upon the strain and temperature of the fiber at the grating. Common fiber Bragg gratings have a reflection peak that changes in wavelength as a function of temperature and strain. Typically, when the fiber Bragg grating is operated with light exhibiting a wavelength of about 1550 nanometers, the response of a fiber Bragg grating to temperature is about 10 picometers per degree Kelvin and the response to strain is about 1.2 picometers per microstrain. Thus, it is difficult from a single measurement to separate temperature effects from strain effects.
In contrast to fiber Bragg gratings, which are discrete devices, Brillouin backscatter is a distributed sensing technique. In this technique, a signal propagated through all parts of an unmodified optical fiber is used to provide a local measure of temperature and strain. The Brillouin backscatter technique exploits the Brillouin interaction between a pulsed optical beam and a continuous wave optical beam counterpropagating in an optical fiber. When the optical frequency of the continuous wave beam is greater than that of the pulsed beam by an amount equal to the Brillouin frequency shift at some point in the fiber, the pulsed beam is amplified through the Brillouin interaction and the continuous wave beam experiences loss. The Brillouin frequency shift and the intensity of the amplification and loss are both strain and temperature sensitive. Using both the intensity and frequency shift information, it is possible to recover the temperature and strain distributions along the optical fiber. Alternatively, it is possible to measure the spontaneous Brillouin backscatter using just a probe pulse.
In certain circumstances, however, it is desirable to use only the frequency shift information, rather than the frequency shift and intensity information, particularly wherein the measurement technique provides only the frequency shift measure of the Brillouin backscattering. In this case, it is necessary to find alternative ways of separating temperature from strain.
One conventional approach of separating temperature from strain is to co-locate two optical fibers, one of which is shielded from strain and the other of which is subjected to strain. Combining measurements from the two fibers provides a means of independently determining temperature and strain. One method of shielding an optical fiber from strain is “loose tube buffering,” wherein the fiber is contained within an outer protective tube such that the fiber can move to some extent. Another method of shielding an optical fiber from strain is the use of a “slotted core,” wherein the optical fiber resides in a channel or groove that has been formed on a surface of a rod-shaped core. Frequently, voids about the fiber are filled with excess fiber. Friction, however, may exist between the optical fiber and the loose tube or slotted core. Moreover, the voids may not be uniformly filled with excess fiber. Accordingly, the optical fiber may not be entirely isolated from strain. Furthermore, strain independence in loose-tube construction is conventionally achieved over a strain range limited to approximately the excess optical fiber length in the tube, which is typically a length of up to 0.7 percent of the cable length. Cable strains greater than this amount will cause strain independence to break down.
While there are many designs of fiber optic cables well known in the art that address cable strain, considerable shortcomings remain.